Adding Langchain example

[pamelafox]

Purpose

Fixes #2 (somewhat) by adding a Langchain example that shows how to switch between the three provider options and do a simple chat completion. I can potentially port the other examples in future to langchain as well if it is useful (or others can).

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Run example for each provider, azure, openai, ollama